SEAWEEDS AND THE WATER-FLORA. 1 77 



matter covers the outside of the diatom. This is pro- 

 duced by a modification of the cellulose ; and its oily or 

 jelly-like nature diminishes friction with the water^ so 

 that the diatom moves readily in any direction. 

 Occasionally, the diatoms become united in strings, 

 which are held together by a common gelatinous coat 

 (see below). 



Diatoms multiply very rapidly. They do so, as a 

 rule, by dividing in two, and each half forming a new 

 diatom. They are said also to reproduce by conjugation, 

 the living matter of two diatoms uniting to form an 

 " Ggg " cell, which then grows and begins to form new 

 diatoms by division. The " Kieselguhr " or diatomite of 

 Skye, Barbadoes, and other places, is an accumulation 

 of the flinty shells of diatoms. It is used for a variety 

 of purposes ; for making fire-proof materials, for making 

 dynamite, and in manufacturing artists' colours. The 

 diatoms contain, when alive, drops of oil w^hich may 

 be regarded as a store of food material. It has been 

 suggested that the shale oil, which is found in certain 

 strata derived from deposits of mud, must have been 

 originally formed by millions of diatoms. Though this 

 is probably, to a certain extent, correct, no doubt both 

 animal and vegetable life contributed to produce the oil. 

 The free-swimming diatoms are not easily collected, 

 but they may be obtained by dredging or trawling 

 with fine muslin nets. The other group of one-cell 

 Algae, the " Peridineae," is not so well known. They 

 also have extraordinary ornaments of flint or calcium ; 

 but they appear in quantities only during certain mont'hs 

 of the year. One of them, Pyrocystis, is phosphorescent, 

 and glows when shaken or irritated by the waves. 



These two groups of Algae supply the wants of 

 most of the minute marine animals, which form the 



(B7) M 



